Most modern wind turbines are controlled and regulated continuously most often with the purpose of ensuring maximum power extraction from the wind under the current wind, weather, while at the same time ensuring that the loads on the different components of the wind turbine are at any time kept within acceptable limits. Desirably, the wind turbine may also be controlled to account for fast local variations in the wind velocity—the so-called wind gusts, and take into account the dynamic changes in the loads on the individual blades due to e.g. the passing of the tower or the actual wind velocity varying with the distance to the ground (the wind profile).
To this purpose a number of parameters are collected and monitored by the controllers in a wind turbine, such as for instance the current wind speed and direction, the wind shear and turbulence, the rotational speed of the rotor, the generator, the pitch angle of each blade, the yaw angle, information on the grid system, and measured parameters (e.g. stresses or vibrations) from sensors placed e.g. on the blades, the nacelle, or on the tower.
Based on these and following some control strategy the optimal control parameters of the turbine in order to perform optimally under the given conditions are determined. The methods of controlling the current performance, and thereby the power production and the load situation of the wind turbine, include for instance pitching of the blades, adjusting any different active aerodynamic devices for changing the aerodynamic surfaces of the blades such as flaps or vortex generating means, adjusting the power, and/or adjusting the rotational speed of the rotor.
Multiple control strategies or ways for the controllers to interact with the wind turbine exist, and may in modern wind turbines be very complex, involving several controllers placed in different parts of the turbine and interacting to continuously determine and update the optimal values of the control parameters. The control strategies may often be based on hundreds of collected and determined parameters in trying to take all eventualities into account and cover both the common, more rare, and extreme operation scenarios.
However, even for very advanced and sophisticated control systems, imperfect or inadequate controlling may still occur for instance due to sensor errors providing the control system with incorrect data, or due to operation scenarios not foreseen or covered by the applied control strategies. Such imperfect controlling may in some control and weather situations result in very high and unacceptable loads in some of the components of the wind turbines, e.g. in the tower due to an undesirable pitching of one or more of the blades, or in the gears due to erroneous adjustments of the power. Such loads may be of considerable sizes and may in the worst case scenario in the extreme situations lead to fatal damage of the turbine. Irrespective that the probability for such extreme situations to arise may be minimal, the possible implications are unacceptable, creating the need for fail-safe control methods capable of preventing these possibly rare but extreme eventualities.